Coherent spin valve and related devices

ABSTRACT

Embodiments of the present invention are directed toward the field of spintronics, and in particular, systems and devices capable of performing spin coherent quantum logic operations. The inventive spin valve comprises two ferromagnetic electrode layers, and a non-magnetic conducting layer positioned therebetween. An external magnetic field {right arrow over (B)} 0  is applied in the Z direction, such that the two electrode layers are each magnetized in a direction substantially parallel to the external magnetic field. Rather than attempting to change the magnetization of one of the ferromagnetic layers, as is the case in prior art technologies, it is the direction of the electron spin that is manipulated in the present embodiments while the electron is traveling through the middle, nonmagnetic layer. One of the ferromagnetic electrodes may be the tip of a scanning tunneling microscope (STM). This configuration may further comprise a bias voltage source connected between the STM tip and the other ferromagnetic electrode, such that a spin polarized tunneling current is conducted between the two.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 60/614,946, by Haitao Yang and Xiao-Dong Xiang, titled“Coherent Spin Valve and Related Devices,” filed Sep. 30, 2004. Thepresent application also claims the benefit of U.S. Provisional PatentApplication No. 60/632,767, by Haitao Yang and Xiao-Dong Xiang, alsotitled “Coherent Spin Valve and Related Devices,” and filed Dec. 1,2004. Both U.S. Provisional Patent Application 60/614,946 and 60/632,767are hereby incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

Embodiments of the present invention are generally related to the fieldof spintronics. In particular, the present embodiments are directed tosystems and devices capable of performing spin coherent quantum logicoperations.

BACKGROUND OF THE INVENTION

Recently, a new technology has emerged known as spintronics. Instead ofconventional charge-based electronic devices, this technology useselectron spin to carry information, and thus offers opportunities for anew generation of spin devices. These devices will have the potentialadvantages of non-volatility, increased data processing speed, decreasedelectric power consumption, and increased integration densities comparedwith conventional semiconductor devices.

The giant magnetoresistance effect (GMR) was discovered in 1988, and isconsidered to be a very promising technology for spintronics devices.GMR is a quantum mechanical effect observed in layered magneticthin-film structures composed of alternating layers of ferromagnetic andnonmagnetic layers. When the magnetizations of the ferromagnetic layersare parallel, the spin dependant scattering of carriers is minimized andthe material has its lowest resistance; in contrast, when themagnetizations are anti-parallel, the spin dependant scattering ofcarriers is maximized and the material displays its highest resistance.The simplest GMR structure is the spin valve, which consists of twoferromagnetic layers sandwiching a thin nonmagnetic conductive layer.The flow of electrons in the spin valve is controlled by changing thedirection of the magnetization of a part of the device. In previous spinvalve structures, one of the two ferromagnetic layers was “pinned” byplacing an anti-ferromagnetic layer in intimate contact with the pinnedlayer. The other ferromagnetic layer was “free” layer whosemagnetization could be changed by applying an external magnetic field.

The basic principle GMR effect may be explained using a simple quantummechanics picture, as illustrated in FIG. 1. In the ferromagnetic layers10, 11A, the electron energy bands of spin up and spin down are split,which results in an unbalanced density of states for spin up and down atFermi level E_(F). In the middle nonmagnetic layer 12, the density ofstates for spin up and spin down are even. The conductivity of the GMRstructure may be expressed as follows:σ∝n _(1↑) n ² n _(2↑) +n _(1↓) n ² n _(2↓)  (1)where n is the density of states of spin up or down in the nonmagneticlayer 12 at the Fermi level, n_(1↑), n_(1↓) is the density of states ofspin up, spin down respectively in the left ferromagnetic electrode(10), and n_(2↑), n_(2↓) is that corresponding to the rightferromagnetic electrode (11A). In FIG. 1A, the magnetizations of the twoelectrodes (the two ferromagnetic layers 10, 11A) are all in the updirection (parallel); the spin up electrons are the majority carriers,and the spin down electrons are the minority carriers. The magnetizationof electrode 11A is reversed to the state shown as electrode 11B in FIG.1B, however, and the spin up, down electrons become the minority,majority carriers, respectively. Assuming n₊, n⁻ as the density ofstates of for the majority, minority cases, respectively, theconductivity of parallel and anti-parallel structures may be written as:σ_(p) ∝n ²(n ₁₊ n ₂₊ +n ¹⁻ n ²⁻)σ_(ap) ∝n ²(n ₁₊ n ²⁻ +n ¹⁻ n ₂₊)  (2)

The spin polarization is defined as P=(n₊−n⁻)/(n₊+n⁻), and giantmagnetoresistance is defined as GMR=(σ_(p)−σ_(ap))/σ_(ap). From equation(2), one may derive:

$\begin{matrix}{{GMR} = \frac{2P_{1}P_{2}}{1 - {P_{1}P_{2}}}} & (3)\end{matrix}$This is same as the Julliere formula for tunneling magneto-resistance.

SUMMARY OF THE INVENTION

In the present embodiments, a coherent electron spin resonancecontrolled spin valve device is implemented. The device utilizes a3-layer sandwich structure, with two ferromagnetic electrode layers anda layer of nonmagnetic conductive material between the electrodes. Theelectron spin relaxation time of the nonmagnetic layer should be longrelative to the conducting electron travel time inside this layer, suchthat the electron spin stay coherent when it moves from one electrode toanother electrode. The spin polarized conduction current between twoferromagnetic electrodes is controlled by an RF or microwave “gate”pulse, which excites the electron spin resonance under a static magneticfield. Quantum phase oscillation behavior can be observed indirect-current conduction of the device as a function of RF (microwave)pulse duration.

In one embodiment of the present invention, the coherent spin valvecomprises a nonmagnetic, electrically conducting layer positionedadjacent to a first ferromagnetic electrode and a second ferromagneticelectrode; a static magnetic field {right arrow over (B)}₀ configured tomagnetize the first and second ferromagnetic electrodes, each directionof magnetization being substantially parallel to the static magneticfield {right arrow over (B)}₀; a radiation source for providing anRF/microwave signal to the nonmagnetic, electrically conducting layer,the RF/microwave signal having a magnetic field {right arrow over (B)}₁which is substantially perpendicular to the a static magnetic field{right arrow over (B)}₀, and configured such that {right arrow over(B)}₁ and {right arrow over (B)}₀ interact with conducting electrons inthe nonmagnetic layer to excite a precessing electron state having aspin resonance frequency, thus generating a spin polarized conductingcurrent; and a detector for monitoring a change in the spin polarizedconducting current. The signal from the RF/microwave radiation source inthe coherent spin valve may be a a continuous wave (CW) signal, or atime varying signal.

In an alternative embodiment of the present invention, the firstferromagnetic electrode of the coherent spin valve comprises the tip ofa scanning tunneling microscope (STM), and this configuration mayinclude a bias voltage source connected between the STM tip and thesecond ferromagnetic electrode such that a spin polarized tunnelingcurrent is conducted between the STM tip and the second ferromagneticelectrode.

In an alternative embodiment of the present invention, a method ofperforming spintronics, is disclosed, the method comprising the steps ofproviding a nonmagnetic, electrically conducting layer positionedadjacent to a first ferromagnetic electrode and a second ferromagneticelectrode; providing a static magnetic field {right arrow over (B)}₀ tomagnetize the first and second ferromagnetic electrodes, each directionof magnetization being substantially parallel to the static magneticfield {right arrow over (B)}₀; providing an RF/microwave signal to thenonmagnetic, electrically conducting layer, the RF/microwave signalhaving a magnetic field {right arrow over (B)}₁ which is substantiallyperpendicular to the a static magnetic field {right arrow over (B)}₀;interacting the magnetic fields {right arrow over (B)}₁ and {right arrowover (B)}₀ with conducting electrons in the nonmagnetic layer to excitea precessing electron state having a spin resonance frequency, thusgenerating a spin polarized conducting current; and detecting a changein the spin polarized conducting current. The method may be a spincoherent quantum logic operation, include a spin echo technique, and/orinclude a 2D Fourier transform electron spin resonance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a schematic diagrams of a simple quantum mechanicsmodel for the GMR effect;

FIG. 2 shows the structure of a simple coherent spin valve;

FIG. 3 is an illustration of electron spin evolution under the verticalmagnetic field {right arrow over (B)}₀ and a horizontal rotating field{right arrow over (B)}₁, observed in {right arrow over (B)}₁ rotatingframe.

FIG. 4 is a coherent spin valve conductivity time resolve curve;

FIG. 5 is a modified coherent spin valve structure, where a polarizedSTM tip functions as one of the magnetic electrodes;

FIG. 6 is a schematic of a planar spin valve structure configured toprovide a local magnetic field and an RF generator; and

FIG. 7 is a schematic of a vertical structure spin valve with an RFsource.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the various embodiments of the presentinvention, the basic structure will be described first, followed by ananalysis of the Bloch equation as it pertains to the presentembodiments. Next, various exemplary applications using the describedprinciples will be provided, followed by a discussion of exemplarymaterials from which the middle nonmagnetic layer and the twoferromagnetic layers may be fabricated.

The Basic Structure

The basic structure is illustrated in FIG. 2, which shows the inventivespin valve comprising two ferromagnetic electrode layers (layers 21A and21C) and a non-magnetic conducting layer (layer 21B) positioned betweenthe two ferromagnetic layers 21A, 21C. An external magnetic field {rightarrow over (B)}₀ is applied in the Z direction, such that the twoelectrode layers 21A, 21C are each magnetized in a directionsubstantially parallel to the external magnetic field {right arrow over(B)}₀.

The inventive concept lies in the following principle: rather thanattempting to change the magnetization of one of the ferromagneticlayers, as is the case in prior art technologies, in the presentembodiments it is the direction of the electron spin that is manipulatedwhile the electron is traveling through the middle, nonmagnetic layer21B. The electrons are injected into layer 21B from layer 21A and moveto layer 21C. Because the electrode layers are magnetized parallel tothe direction of external magnetic field {right arrow over (B)}₀, thespin of the electrons ejected to the middle layer 21B from layer 21A areinitially polarized in a direction parallel to {right arrow over (B)}₀.If the direction of the electron spin is maintained until the reachesthe opposite end of the layer, the spin direction will be parallel tothe magnetization direction of electrode 21C, and the device will yielda high conductivity. If the spin direction has been reversed during thetransmission through layer 21B, however, the conductivity will be low.

The manipulation of electron spins may be realized using electron spinresonance techniques. The ferromagnetic layers 21A, 21C in the spinvalve structure may be replaced by half metallic materials orsemiconductor materials whose spin polarization is achieved by opticalpumping or other polarization technologies. A radio frequency (RF)signal 22 may be applied to the layer 21B, where the RF signal 22 may beeither a continuous wave (CW) or pulsed signal, such that the RFmagnetic field {right arrow over (B)}₁ is perpendicular to the externalmagnetic field {right arrow over (B)}₀, and rotating around {right arrowover (B)}₀ in the Larmor frequency governed by the relationshiphv=gμ_(B)B₀. In this case, the polarized electrons injected fromelectrode 21A into the layer 21B will have a spin resonance, and theelectron spins can precess both vertically and horizontally. Dependingon the applied RF signal 22, the electron spins in the layer 21B can bemanipulated to be at any state. A detailed analysis of the spinresonance assumed by the electrons is provided using the classic Blochequation.

Bloch Equation Analysis

Assuming {right arrow over (B)}₀ is in the {right arrow over (e)}_(z)direction: {right arrow over (B)}₀=B₀{right arrow over (e)}_(z); and RFmagnetic field {right arrow over (B)}₁ is initially along {right arrowover (e)}_(x) and rotating around {right arrow over (B)}₀ clockwise inthe angular velocity ω:{right arrow over (B)} ₁={right arrow over (e)}_(x) B ₁ cos(ωt)−{rightarrow over (e)}_(y) B ₁ sin(ωt)The initial magnetic moment of electron spins is at its thermalequilibrium value {right arrow over (M)}₀ which is along {right arrowover (e)}_(z) also: {right arrow over (M)}₀=M₀{right arrow over(e)}_(z).

The spin movement under magnetic field is governed by Bloch equation:

$\begin{matrix}{\frac{\mathbb{d}\overset{\rightharpoonup}{M}}{\mathbb{d}t} = {{\gamma\;\overset{\rightharpoonup}{M} \times \overset{\rightharpoonup}{B}} - {\left( {{M_{x}{\overset{\rightharpoonup}{e}}_{x}} + {M_{y}{\overset{\rightharpoonup}{e}}_{y}}} \right)/T_{2}} + {{{\overset{\rightharpoonup}{e}}_{z}\left( {M_{0} - M_{z}} \right)}/T_{1}}}} & (1)\end{matrix}$where γ is gyromagnetic ratio γ=gμ_(B)/

, and {right arrow over (B)}={right arrow over (B)}₀+{right arrow over(B)}₁. T₁ and T₂ are the spin-lattice and spin-spin relaxation times,respectively. Written in matrix form, equation (1) becomes:

$\begin{matrix}{\frac{\mathbb{d}\overset{\rightharpoonup}{M}}{\mathbb{d}t} = {{\;\overset{\rightharpoonup}{M}} + {{{\overset{\rightharpoonup}{M}}_{0}/T_{1}}\mspace{25mu}{and}}}} & (2) \\{{= \begin{bmatrix}{{- 1}/T_{2}} & {\gamma\; B_{0}} & {\gamma\; B_{1}\sin\;\omega\; t} \\{{- \gamma}\; B_{0}} & {{- 1}/T_{2}} & {\gamma\; B_{1}\cos\;\omega\; t} \\{{- \gamma}\; B_{1}\sin\;\omega\; t} & {{- \gamma}\; B_{1}\cos\;\omega\; t} & {{- 1}/T_{1}}\end{bmatrix}},{{\overset{\rightharpoonup}{M}}_{0} = \begin{pmatrix}0 \\0 \\M_{0}\end{pmatrix}}} & (3)\end{matrix}$By introducing a rotating reference frame which rotates around the Zaxis clockwise, and with frequency ω, the Bloch equation can be greatlysimplified. Defining{right arrow over (M)}′=U⁻¹{right arrow over (M)}  (4)U⁻¹ is the anticlockwise transformation matrix:

$U^{- 1} = \begin{bmatrix}{\cos\;\omega\; t} & {{- \sin}\;\omega\; t} & 0 \\{\sin\;\omega\; t} & {\cos\;\omega\; t} & 0 \\0 & 0 & 1\end{bmatrix}$Substituting equation (4) into equation (2), the following equation isobtained:

$\begin{matrix}{\frac{\mathbb{d}{\overset{\rightharpoonup}{M}}^{\prime}}{\mathbb{d}t} = {{\Theta\;{\overset{\rightharpoonup}{M}}^{\prime}} + {{{\overset{\rightharpoonup}{M}}_{0}/T_{1}}\mspace{25mu}{where}}}} & (5) \\\begin{matrix}{\Theta = {{U^{- 1}U} + {\frac{\mathbb{d}U^{- 1}}{\mathbb{d}t}U}}} \\{= \begin{bmatrix}{{- 1}/T_{2}} & {\Delta\omega} & 0 \\{- {\Delta\omega}} & {{- 1}/T_{2}} & \omega_{1} \\0 & {- \omega_{1}} & {{- 1}/T_{1}}\end{bmatrix}}\end{matrix} & (6)\end{matrix}$and ω₀=γB₀, ω₁=γB₁, Δω=ω₀−ω.

The solution to the differential equation (5) is:{right arrow over (M)}′(t)=exp(Θt){right arrow over (M)}′ ₀−Θ⁻¹ {rightarrow over (M)} ₀ /T ₁  (7)and{right arrow over (M)}′ ₀ ={right arrow over (M)} ₀+Θ⁻¹ {right arrowover (M)} ₀ /T ₁The calculation of exp(Θt) is very complicated. To simplify thecalculation and get a rough picture, we assume that T₁=T₂=τ, then,

$\begin{matrix}{{\exp\left( {\Theta\; t} \right)} = {\frac{{\mathbb{e}}^{{- t}/\tau}}{\omega_{1}^{\prime 2}}\begin{bmatrix}{\omega_{1}^{2} + {{\Delta\omega}^{2}\cos\;\omega_{1}^{\prime}t}} & {{\Delta\omega\omega}_{1}^{\prime}\sin\;\omega_{1}^{\prime}t} & {{\Delta\omega\omega}_{1}\left( {1 - {\cos\;\omega^{\prime}t}} \right)} \\{{- {\Delta\omega\omega}_{1}^{\prime}}\sin\;\omega_{1}^{\prime}t} & {\omega_{1}^{\prime 2}\cos\;\omega_{1}^{\prime}t} & {\omega_{1}\omega_{1}^{\prime}\sin\;\omega_{1}^{\prime}t} \\{{\Delta\omega\omega}_{1}\left( {1 - {\cos\;\omega^{\prime}t}} \right)} & {{- \omega_{1}}\omega_{1}^{\prime}\sin\;\omega_{1}^{\prime}t} & {{\Delta\omega}^{2} + {\omega_{1}^{2}\cos\;\omega_{1}^{\prime}t}}\end{bmatrix}}} & (8)\end{matrix}$and ω′₁ is defined as ω′₁ ²=ω₁ ²+Δω².

The spin moment in the rotating frame is then

$\begin{matrix}{{{\overset{\rightharpoonup}{M}}^{\prime}(t)} = {{\frac{M_{0}\omega_{1}\tau\;{\mathbb{e}}^{{- t}/\tau}}{\sqrt{1 + {\omega_{1}^{\prime 2}\tau^{2}}}}\begin{pmatrix}{{- \frac{\Delta\omega}{\omega_{1}^{\prime}}}{\cos\left( {{\omega_{1}^{\prime}t} - \alpha} \right)}} \\{\sin\left( {{\omega_{1}^{\prime}t} - \alpha} \right)} \\{\frac{\omega_{1}}{\omega_{1}^{\prime}}{\cos\left( {{\omega_{1}^{\prime}t} - \alpha} \right)}}\end{pmatrix}} + {\frac{M_{0}}{1 + {\omega_{1}^{\prime 2}\tau^{2}}}\begin{pmatrix}{{\Delta\omega\omega}_{1}\tau^{2}} \\{\omega_{1}\tau} \\{1 + {{\Delta\omega}^{2}\tau^{2}}}\end{pmatrix}}}} & (9)\end{matrix}$and the spin moment in the absolute frame is

$\begin{matrix}{{\overset{\rightharpoonup}{M}(t)} = {{\frac{M_{0}\omega_{1}{\tau\mathbb{e}}^{{- t}/\tau}}{\sqrt{1 + {\omega_{1}^{\prime 2}\tau^{2}}}}\begin{pmatrix}{{{\sin\left( {{\omega_{1}^{\prime}t} - \alpha} \right)}\sin\;\omega\; t} - {\frac{\Delta\omega}{\omega_{1}^{\prime}}{\cos\left( {{\omega_{1}^{\prime}t} - \alpha} \right)}\cos\;\omega\; t}} \\{{{\sin\left( {{\omega_{1}^{\prime}t} - \alpha} \right)}\cos\;\omega} + {\frac{\Delta\omega}{\omega_{1}^{\prime}}{\cos\left( {{\omega_{1}^{\prime}t} - \alpha} \right)}\sin\;\omega\; t}} \\{\frac{\omega_{1}}{\omega_{1}^{\prime}}{\cos\left( {{\omega_{1}^{\prime}t} - \alpha} \right)}}\end{pmatrix}} + {\frac{M_{0}}{1 + {\omega_{1}^{\prime 2}\tau^{2}}}\begin{pmatrix}{\omega_{1}{\tau\left( {{{\Delta\omega\tau}\;\cos\;\omega\; t} + {\sin\;\omega\; t}} \right)}} \\{\omega_{1}{\tau\left( {{\cos\;\omega\; t} - {{\Delta\omega\tau}\;\sin\;\omega\; t}} \right)}} \\{1 + {{\Delta\omega}^{2}\tau^{2}}}\end{pmatrix}}}} & (10)\end{matrix}$α is defined as

${\cos\;\alpha} = {{\frac{\omega_{1}^{\prime}\tau}{\sqrt{1 + {\omega_{1}^{\prime 2}\tau^{2}}}}\mspace{14mu}{and}\mspace{14mu}\sin\;\alpha} = {\frac{1}{\sqrt{1 + {\omega_{1}^{\prime 2}\tau^{2}}}}.}}$

From equation (9), one skilled in the art will note that the spin momentin the rotating frame consists of two terms: a first oscillating decayterm and a second constant term. The oscillating part of the spin momentin the rotating frame will decay to zero after a sufficient period oftime, which leaves the constant portion of the spin moment as the finalstate in which the spin precessing is balanced with thermal relaxation.

The next part of the analysis considers the situation where the RFfrequency ω is equal to the spin resonant frequency; i.e., Δω=0 andω′₁=ω₁. Equation (9) then becomes:

$\begin{matrix}{{{\overset{\rightharpoonup}{M}}^{\prime}(t)} = {{M_{0}{\mathbb{e}}^{{- t}/\tau}\cos\;{\alpha\begin{pmatrix}0 \\{\sin\left( {{\omega_{1}t} - \alpha} \right)} \\{\cos\left( {{\omega_{1}t} - \alpha} \right)}\end{pmatrix}}} + {M_{0}\sin\;{\alpha\begin{pmatrix}0 \\{\cos\;\alpha} \\{\sin\;\alpha}\end{pmatrix}}}}} & (11)\end{matrix}$and α will become 0 if τ−>∞ (in other words, if the relaxation timeapproaches an infinite length of time).

The behavior of equation (11) is illustrated in FIG. 3. FIG. 3 is anillustration of the electron spin evolution under the vertical magneticfield {right arrow over (B)}₀ and a horizontal rotating field {rightarrow over (B)}₁, observed in the {right arrow over (B)}₁ rotating timeframe. The illustrated curve is calculated under the condition ω=ω₀ andω₁τ=10. If relaxation time τ is long enough, the spin moment is justsimply rotating along the X axis with an angular velocity ω₁, and wherea decay is observed when the observation is made in the rotating frame.

Next, various exemplary applications using the presently describedprinciples will be provided.

Exemplary Applications

From equation (11), one skilled in the art will note that the angle ofthe spin rotation along horizontal axis is ω₁t. By controlling the RFpulse time t or magnetic field B₁, the spin rotation angle may bemanipulated, adjusted and/or controlled, and therefore the spin verticaland the horizontal components may also be manipulated, adjusted, and/orcontrolled. In this manner, the resistance of the spin valve may beadjusted.

Based on the GMR principle, the spin valve conductance is actuallydetermined by the Z component (parallel to the magnetization direction)of the electron spins that reach the interface between nonmagnetic layer21B and the magnetic layer 21C. FIG. 4 gives an example of the decayedoscillation behavior of the spin valve conductance with respect to theRF pulse width, or a “coherent spin valve conductivity time resolvecurve.” The time is either the RF pulse duration or electrontransporting time in the nonmagnetic layer 21B, whichever is shorter; inother words, time t is either the pulse width of the RF signal 22, orthe electron transport time within the middle layer 21B if thattransport time is shorter than the pulse width of the RF signal 22. Theremaining conditions are the same as those of FIG. 3, including thecondition that ω₁τ=10.

Another exemplary structure based on the scheme depicted in FIG. 2 maybe based on the same principle is shown in FIG. 5, which illustrates thesituation where one of the magnetic layers of the coherent spin valve(what may have been layer 21A in FIG. 2) has been replaced by a spinpolarized scanning tunneling microscope (STM) tip 51. When a biasvoltage 52 applied between the STM tip 51 and the magnetic layer 21C, aspin polarized tunneling current can occur between the STM tip 51 andthe nonmagnetic middle layer 21B. The tunneled electrons may then bemanipulated by RF pulses (again, by an RF signal 22) before theelectrons reach the magnetic layer 21C. The structure in FIG. 5 makes itpossible to carry out a single electron operation.

Conventional pulsed ESR techniques known in the art may be combined withthe inventive coherent spin valve to enhance detection sensitivity, andadd new functionality such as the spin echo technique, or 2D Fouriertransform electron spin resonance. Although the spin valve resistance isnot sensitive to horizontal spin components, which are perpendicular tothe magnetization direction, a subsequent π/2 RF pulse will turn thehorizontal spin components toward the vertical direction. This puts themin a condition that the spin valve can detect. In addition to theconventional π/2-t-π RF pulse sequence in the spin echo technique, forexample, a subsequent π/2 RF pulse is needed to enable the spin valvedetection (π/2-t₁-π-t₂-π/2). The signal strength will be determined bythe time in which the subsequent pulse is delayed from the spin echopulse sequence (t₂), instead of the detection time in a conventionalspin echo measurement. For 2D-FT-ESR measurement, aπ/2-t₁-π/2-τ-π/2-t₂-π/2 pulse sequence is useful.

The resistance (or conductance) of the spin valve is actually the directresult of a quantum interaction between the electron spins, theRF/microwave signal, and the external magnetic field. The embodimentsand principles described herein actually comprise quantum computingdevice(s), where the RF signal is functions as a gate. The RF signal maybe provided by an RF coil or micro-coil, micro-strip line, or RFresonator/cavity.

In some embodiments, the spin valve may be configured as planarstructure. As illustrated in FIG. 6, the spin valve in thisconfiguration comprises a ferromagnetic layer 61, nonmagnetic layer 62,and a ferromagnetic layer 63, each of which lie in a plane. The staticmagnetic field may be provided by a multi-domain magnetic nano-wire orsimilar structure, shown generally in FIG. 3 as a nano-structure 64having domains A and B. The fringe field from the domain wall betweendomains A and B provides the static magnetic field to the nonmagneticlayer 62 of the spin valve. The direction of this fringe field is withinthe plane of the structure, and perpendicular to the direction that thecurrent is flowing. The magnetization directions of the twoferromagnetic layers 61 and 63 of the spin valve are indicated in theFIG. 6.

If one of the ferromagnetic layers is pinned (say, for example,ferromagnetic layer 61), the magnetization of the second layer, in thiscase layer 63, is free to change. The free layer magnetization (in otherwords, the direction of the magnetization of the layer 63) will bealigned parallel to the fringe field. By switching the fringe field, themagnetization of the free ferromagnetic layer 63 can be easily switched.The switching of the fringe field can be achieved by controlling thedomain wall position (the interface between domains A and B of themulti-domain magnetic structure 64) with electric current flowingthrough the domain wall. The switching of the free magnetic layermagnetization will add more function to the spin valve logic operation.

The RF source may be provided by a spin momentum transfer (SMT)oscillator 65 placed on top of the spin valve. The SMT 65 device maycomprise a bottom electrode layer 65A, a fixed ferromagnetic layer 65B,a conductive layer 65C, a free ferromagnetic layer 65D, and a topelectrode layer 65E, as illustrated in FIG. 6. When a DC current flowsthrough the SMT device 65, an oscillation will result on top of the DCcurrent. The oscillation frequency is in an RF or microwave frequencyrange, and it may be tuned by manipulating the level of the DC current.A micro-coil 66 is then connected to the bottom electrode 65A, whichproduces an oscillating magnetic field in the nonmagnetic layer 62 ofthe spin valve.

An SMT oscillator may also be used as the RF source in the generalizedlayered structure shown in FIG. 2. Such a configuration is shown in FIG.7. The exemplary spin valve shown in FIG. 7 comprises a topferromagnetic layer 71, which may be either the left or right layeroriginally shown in FIG. 2. In FIG. 2, the two ferromagnetic layers 21A,21C were substantially the same size. In the alternative embodimentshown in FIG. 7, however, the top ferromagnetic layer 71 is configuredto be smaller than either the middle, nonmagnetic conductive layer 72,or the bottom ferromagnetic layer 73, such that the RF field produced bySMT micro-coil 74 can penetrate into the nonmagnetic middle layer 72.

Materials

The materials from which the nonmagnetic layer of the spin valvenonmagnetic layer may be fabricated may either be semiconductive, ormade from some conductive material featuring a long electron spinrelaxation time. Graphite is one example.

The materials in the ferromagnetic layers may be ferromagnetic materialssuch as Co, half metallic materials with high spin polarization, such asCrO₂, Sr₂FeMoO₆ and La_(1-x)Sr_(x)MnO₃, dilute magnetic semiconductorssuch as (Ga, Mn)As and (Hg, Mn)Se, half Heusler alloys such as NiMnSb,Mn₂VAl, Heusler alloys such as Ag₂MgCd, Ag₂MgIn, Ag₂MgZn, AuAgZn₂,AuCuZn₂, Ce₁₄Mg₄₃H₄₃, Ce₂₅Ag₂₅Cu₂₅In₂₅, Ce₂LaIn, CeAg₂In, CeAu₂In,CeCu₂In, Cr₁₀Mn₁₅Ni₅₀Sn₂₅, Cr₁₅Mn₁₀Ni₅₀Sn₂₅, CrCo₂Al, CrCo₂Ga, CrFe₂Al,CrFe₂Ga, CrNi₂Al, DyAg₂In, DyAu₂In, DyCu₂In, DyPd₂Bi, DyPd₂In, DyPd₂Pb,DyPd₂Sb, DyPd₂Sn, ErAg₂In, ErAu₂In, ErCu₂In, ErPd₂Bi, ErPd₂In, ErPd₂Pb,ErPd₂Sb, ErPd₂Sn, Fe₂₁Ni₉Si₁₀, Fe₂CoGa, Fe₂CoGe, Fe₂NiAl, Fe₂NiGa,Fe₉Co₆Ga₅, Fe₉Ni₆Ga₅, FeCo₂Al, FeCo₂Ga, FeCo₂Ge, FeCo₂In, FeCo₂Si,FeRu₂Si, FeRu₂Sn, GdAg₂In, GdAu₂In, GdCu₂In, GdPd₂Bi, GdPd₂In, GdPd₂Pb,GdPd₂Sb, HfAu₂Al, HfAu₂In, HfCo₂Al, HfCo₂Ga, HfCo₂Sn, HfCu₂Al, HfNi₂Al,HfNi₂Ga, HfNi₂Sn, HoAg₂In, HoAu₂In, HoCu₂In, HoPd₂Bi, HoPd₂Pb, HoPd₂Sb,LaAg₂In, LaCu₂In, Li₂₀ZrAs₈, Li₂₀ZrBi₈, Li₂₀ZrP₈, Li₂₀ZrSb₈,Li₂₂Ti₂As₁₀, Li₂₂Ti₂Bi₁₀, Li₂₂Ti₂P₁₀, Li₂₂Ti₂Sb₁₀, Li₂₆HfAs₁₀,Li₂₆HfBi₁₀, Li₂₆HfSb₁₀, Li₂IrSn, Li₂MgIn, Li₂MgPb, Li₂MgTl, Li₂PdGe,Li₂PdPb, Li₂PdSn, Li₂PtSn, Li₂ZnGe, LiCo₂Ge, LiIrAl₂, LiIrGa₂, LiIrIn₂,LiMg₂Ga, LiMg₂Ge, LiNi₂Sn, LiPd₂Ge, LiPd₂Sn, LiPdAl₂, LiPdGa₂, LiPdIn₂,LiPtAl₂, LiPtGa₂, LiPtIn₂, LiRhAl₂, LiRhGa₂, LiRhIn₂, LiRuGa₂, LiRuIn₂,LuAu₂In, LuCu₂In, LuNi₂Sn, LuPd₂In, LuPd₂Sn, Mn₂₅Co₂₅Cu₂₅Sn₂₅,Mn₂₅Ni₂₅Cu₂₅Sn₂₅, Mn₂₅Ni₅₀In₄Sn₂₁, Mn₂₅Pd₂₀Cu₃₀Al₂₅, Mn₂₅Pd₅₀In₁₂Sb₁₃,Mn₂CoSn, Mn₂Cu₄InSn, Mn₃Cu₃Al₂, Mn₃Fe₄Ga₉, Mn₄Pd₈In_(1.92)Sn_(2.08)Cf₁₆,Mn₄Pd₈Sn_(2.08)Sb_(1.92)Cf₁₆, MnAu₂Al, MnCo₂Al, MnCo₂Ga, MnCo₂Ge,MnCo₂Sb, MnCo₂Si, MnCo₂Sn, MnCoSb, MnCu₂Al, MnCu₂In, MnCu₂Sb, MnCu₂Sn,MnFe₂Al, MnFe₂Si, MnNi₂Al, MnNi₂Ga, MnNi₂Ge, MnNi₂In, MnNi₂Sb, MnNi₂Sn,MnPd₂Al, MnPd₂Ge, MnPd₂In, MnPd₂Sb, MnPd₂Sn, MnPt₂Al, MnRh₂Ge, MnRh₂Pb,MnRh₂Sn, MoFe₂Al, NaLi₂Sb, NbCo₂Al, NbCo₂Ga, NbCo₂Sn, NbNi₂Al, NbNi₂Ga,NbNi₂Sn, NbRu₂Al, NdAg₂In, NdAu₂In, NdCu₂In, Ni₂CuSb, Ni₂CuSn, Ni₂MgIn,Ni₂MgSb, Ni₂MgSn, Ni₆Cu₉Sb₅, Ni₆Cu₉Sn₅, NiCu₂Sn, Pd₂MgGa, Pd₂MgIn,PrAg₂In, PrAu₂In, PrCu₂In, PrPd₂Bi, PrPd₂Pb, Rh₂CuSn, Rh₂NiSn, ScAg₂Al,ScAg₂Al, ScAg₂In, ScAu₂Al, ScAu₂In, ScCo₂Sn, ScCoNiSn, ScCu₂Al, ScCu₂Ga,ScCu₂In, ScNi₂Al, ScNi₂Ga, ScNi₂In, ScNi₂Sn, ScPd₂Al, ScPd₂Ga, ScPd₂In,ScPd₂Sn, ScPt₂In, ScPt₂Sn, SmAg₂In, SmAu₂In, SmCu₂In, SmPd₂Bi, SmPd₂Pb,TaCo₂Al, TbAg₂In, TbAu₂In, TbCu₂In, TbPd₂Bi, TbPd₂Pb, TbPd₂Sn,Ti₁₅Mn₁₀Ni₅₀Sn₂₅, Ti₂₅Fe₅₀Al₂₅, TiAu₂Al, TiAu₂In, TiCo₂Al, TiCo₂Ga,TiCo₂Ge, TiCo₂Si, TiCo₂Sn, TiCoNiSn, TiCu₂Al, TiCu₂In, TiFe₂Al, TiFe₂Ga,TiFe₂Sn, TiNi₂Al, TiNi₂Ga, TiNi₂In, TiNi₂Sb, TiNi₂Sn, TiPd₂In, TmAg₂In,TmAu₂In, TmCu₂In, TmPd₂In, TmPd₂Sn, UNi₂In, UNi₂Sn, V₁₅Mn₁₀Ni₅₀Sn₂₅,V₁₉Fe₅₆Si₂₅, V₂₅Fe₅₀Al₂₅, V₂₅Fe₅₀Si₂₅, V₃Fe₁₂Ge₅, V6Fe₉Ga₅, V₉Fe₂₁Si₁₀,VCo₂Al, VCo₂Ga, VCo₂Si, VCo₂Sn, VFe₂Al, VFe₂Ga, VFe₂Si, VFe₂Sn, VMn₂Al,VMn₂Ga, VMnCo₄Si₂, VMnFe₄Si₂, VNi₂Al, VNi₂Ga, VNi₂Sn, VRh₂Sn, WMn₂Sn,Y_(0.25)Ce_(0.75)Au₂In, Y_(0.5)Pr_(0.5)Mg₃, Y₂₅Pd₂₅Au₂₅In₂₅, Y₂Mg₃Zn₃,YAg₂In, YAu₂In, YCd₃, YCe₃Au₈In₄, YCu₂In, YLaMg₆, YPd₂Bi, YPd₂In,YPd₂Pb, YPd₂Sb, YPd₂Sn, YbAu₂In, YbNi₂Sn, YbPd₂Pb, YbPd₂Sb, YbPd₂Sn,ZrAu₂In, ZrCo₂Al, ZrCo₂Sn, ZrCu₂Al, ZrNi₂Al, ZrNi₂In, ZrNi₂Sb, ZrNi₂Sn,and ZrPt₂In.

To increase the spin injection efficiency between the ferromagneticlayer and the nonmagnetic layer in spin valve, a buffer layer may beinserted between the two. An example of a buffer layer is an oxide.

Many modifications of the exemplary embodiments of the inventiondiscussed above may readily occur to those skilled in the art.Accordingly, the invention is to be construed as including all thestructures and methods that fall within the scope of the appendedclaims.

1. A coherent spin valve comprising: a nonmagnetic, electricallyconducting layer positioned adjacent to a first ferromagnetic electrodeand a second ferromagnetic electrode; a static magnetic field {rightarrow over (B)}₀ configured to magnetize the first and secondferromagnetic electrodes, each direction of magnetization beingsubstantially parallel to the static magnetic field {right arrow over(B)}₀; a radiation source for providing an RF/microwave signal to thenonmagnetic, electrically conducting layer, the RF/microwave signalhaving a magnetic field {right arrow over (B)}₁ which is substantiallyperpendicular to the static magnetic field {right arrow over (B)}₀, andconfigured such that {right arrow over (B)}₁ and {right arrow over (B)}₀interact with conducting electrons in the nonmagnetic layer to excite aprecessing electron state having a spin resonance frequency, thusgenerating a spin polarized conducting current; and a detector formonitoring a change in the spin polarized conducting current.
 2. Thecoherent spin valve of claim 1, wherein the signal from the RF/microwaveradiation source is a continuous wave (CW) signal.
 3. The coherent spinvalve of claim 1, wherein the signal from the RF/microwave radiationsource is a time varying signal.
 4. The coherent spin valve of claim 3,wherein the time varying signal contains at least one pulse such thatthe excited electron spin precessing angle is controlled by adjustingthe pulse width.
 5. The coherent spin valve of claim 1, wherein theRF/microwave radiation source is selected from the group consisting ofan RF coil, a micro-coil, a micro-strip line, or an RF cavity.
 6. Thecoherent spin valve of claim 1, wherein the RF/microwave radiationsource is a spin momentum transfer (SMT) oscillator.
 7. The coherentspin valve of claim 1, wherein the first ferromagnetic electrodecomprises the tip of a scanning tunneling microscope (STM).
 8. Thecoherent spin valve of claim 7, further including a bias voltage sourceconnected between the STM tip and the second ferromagnetic electrode,such that a spin polarized tunneling current is conducted between theSTM tip and the second ferromagnetic electrode.
 9. The coherent spinvalve of claim 8, wherein the tunneling current is controlled to allowsubstantially single electron operation.
 10. The coherent spin valve ofclaim 1, wherein the nonmagnetic, electrically conducting layer, thefirst ferromagnetic electrode, and the second ferromagnetic electrodeare co-planar.
 11. The coherent spin valve of claim 10, wherein thestatic magnetic field {right arrow over (B)}₀ is provided by amulti-domain magnetic nano-wire structure.
 12. The coherent spin valveof claim 11, wherein the direction of magnetization of the firstferromagnetic electrode is pinned, and the direction of magnetization ofthe second ferromagnetic electrode is free to change.
 13. The coherentspin valve of claim 1, wherein at least one of the ferromagneticelectrodes is Co.
 14. The coherent spin valve of claim 1, wherein atleast one of the ferromagnetic layers is a half metallic material. 15.The coherent spin valve of claim 14, wherein the half metallic materialis selected from the group consisting of CrO₂, Sr₂FeMoO₆,La_(1-x)Sr_(x)MnO₃, NiMnSb, Mn₂VAl, Ag₂MgCd, Ag₂MgIn, Ag₂MgZn, AuAgZn₂,AuCuZn₂, Ce₁₄Mg₄₃H₄₃, Ce₂₅Ag₂₅Cu₂₅In₂₅, Ce₂LaIn, CeAg₂In, CeAu₂In,CeCu₂In, Cr₁₀Mn₁₅Ni₅₀Sn₂₅, Cr₁₅Mn₁₀Ni₅₀Sn₂₅, CrCo₂Al, CrCo₂Ga, CrFe₂Al,CrFe₂Ga, CrNi₂Al, DyAg₂In, DyAu₂In, DyCu₂In, DyPd₂Bi, DyPd₂In, DyPd₂Pb,DyPd₂Sb, DyPd₂Sn, ErAg₂In, ErAu₂In, ErCu₂In, ErPd₂Bi, ErPd₂In, ErPd₂Pb,ErPd₂Sb, ErPd₂Sn, Fe₂₁Ni₉Si₁₀, Fe₂CoGa, Fe₂CoGe, Fe₂NiAl, Fe₂NiGa,Fe₉Co₆Ga₅, Fe₉Ni₆Ga₅, FeCo₂Al, FeCo₂Ga, FeCo₂Ge, FeCo₂In, FeCo₂Si,FeRu₂Si, FeRu₂Sn, GdAg₂In, GdAu₂In, GdCu₂In, GdPd₂Bi, GdPd₂In, GdPd₂Pb,GdPd₂Sb, HfAu₂Al, HfAu₂In, HfCo₂Al, HfCo₂Ga, HfCo₂Sn, HfCu₂Al, HfNi₂Al,HfNi₂Ga, HfNi₂Sn, HoAg₂In, HoAu₂In, HoCu₂In, HoPd₂Bi, HoPd₂Pb, HoPd₂Sb,LaAg₂In, LaCu₂In, Li₂₀ZrAs₈, Li₂₀ZrBi₈, Li₂₀ZrP₈, Li₂₀ZrSb₈,Li₂₂Ti₂As₁₀, Li₂₂Ti₂Bi₁₀, Li₂₂Ti₂P₁₀, Li₂₂Ti₂Sb₁₀, Li₂₆HfAs₁₀,Li₂₆HfBi₁₀, Li₂₆HfSb₁₀, Li₂IrSn, Li₂MgIn, Li₂MgPb, Li₂MgTl, Li₂PdGe,Li₂PdPb, Li₂PdSn, Li₂PtSn, Li₂ZnGe, LiCo₂Ge, LiIrAl₂, LiIrGa₂, LiIrIn₂,LiMg₂Ga, LiMg₂Ge, LiNi₂Sn, LiPd₂Ge, LiPd₂Sn, LiPdAl₂, LiPdGa₂, LiPdIn₂,LiPtAl₂, LiPtGa₂, LiPtIn₂, LiRhAl₂, LiRhGa₂, LiRhIn₂, LiRuGa₂, LiRuIn₂,LuAu₂In, LuCu₂In, LuNi₂Sn, LuPd₂In, LuPd₂Sn, Mn₂₅Co₂₅Cu₂₅Sn₂₅,Mn₂₅Ni₂₅Cu₂₅Sn₂₅, Mn₂₅Ni₅₀In₄Sn₂₁, Mn₂₅Pd₂₀Cu₃₀Al₂₅, Mn₂₅Pd₅₀In₁₂Sb₁₃,Mn₂CoSn, Mn₂Cu₄InSn, Mn₃Cu₃Al₂, Mn₃Fe₄Ga₉, Mn₄Pd₈In_(1.92)Sn_(2.08)Cf₁₆,Mn₄Pd₈Sn_(2.08)Sb_(1.92)Cf₁₆, MnAu₂Al, MnCo₂Al, MnCo₂Ga, MnCo₂Ge,MnCo₂Sb, MnCo₂Si, MnCo₂Sn, MnCoSb, MnCu₂Al, MnCu₂In, MnCu₂Sb, MnCu₂Sn,MnFe₂Al, MnFe₂Si, MnNi₂Al, MnNi₂Ga, MnNi₂Ge, MnNi₂In, MnNi₂Sb, MnNi₂Sn,MnPd₂Al, MnPd₂Ge, MnPd₂In, MnPd₂Sb, MnPd₂Sn, MnPt₂Al, MnRh₂Ge, MnRh₂Pb,MnRh₂Sn, MoFe₂Al, NaLi₂Sb, NbCo₂Al, NbCo₂Ga, NbCo₂Sn, NbNi₂Al, NbNi₂Ga,NbNi₂Sn, NbRu₂Al, NdAg₂In, NdAu₂In, NdCu₂In, Ni₂CuSb, Ni₂CuSn, Ni₂MgIn,Ni₂MgSb, Ni₂MgSn, Ni₆Cu₉Sb₅, Ni₆Cu₉Sn₅, NiCu₂Sn, Pd₂MgGa, Pd₂MgIn,PrAg₂In, PrAu₂In, PrCu₂In, PrPd₂Bi, PrPd₂Pb, Rh₂CuSn, Rh₂NiSn, ScAg₂Al,ScAg₂In, ScAu₂Al, ScAu₂In, ScCo₂Sn, ScCoNiSn, ScCu₂Al, ScCu₂Ga, ScCu₂In,ScNi₂Al, ScNi₂Ga, ScNi₂In, ScNi₂Sn, ScPd₂Al, ScPd₂Ga, ScPd₂In, ScPd₂Sn,ScPt₂In, ScPt₂Sn, SmAg₂In, SmAu₂In, SmCu₂In, SmPd₂Bi, SmPd₂Pb, TaCo₂Al,TbAg₂In, TbAu₂In, TbCu₂In, TbPd₂Bi, TbPd₂Pb, TbPd₂Sn, Ti₁₅Mn₁₀Ni₅₀Sn₂₅,Ti₂₅Fe₅₀Al₂₅, TiAu₂Al, TiAu₂In, TiCo₂Al, TiCo₂Ga, TiCo₂Ge, TiCo₂Si,TiCo₂Sn, TiCoNiSn, TiCu₂Al, TiCu₂In, TiFe₂Al, TiFe₂Ga, TiFe₂Sn, TiNi₂Al,TiNi₂Ga, TiNi₂In, TiNi₂Sb, TiNi₂Sn, TiPd₂In, TmAg₂In, TmAu₂In, TmCu₂In,TmPd₂In, TmPd₂Sn, UNi₂In, UNi₂Sn, V₁₅Mn₁₀Ni₅₀Sn₂₅, V₁₉Fe₅₆Si₂₅,V₂₅Fe₅₀Al₂₅, V₂₅Fe₅₀Si₂₅, V₃Fe₁₂Ge₅, V6Fe₉Ga₅, V₉Fe₂₁Si₁₀, VCo₂Al,VCo₂Ga, VCo₂Si, VCo₂Sn, VFe₂Al, VFe₂Ga, VFe₂Si, VFe₂Sn, VMn₂Al, VMn₂Ga,VMnCo₄Si₂, VMnFe₄Si₂, VNi₂Al, VNi₂Ga, VNi₂Sn, VRh₂Sn, WMn₂Sn,Y_(0.25)Ce_(0.75)Au₂In, Y_(0.5)Pr_(0.5)Mg₃, Y₂₅Pd₂₅Au₂₅In₂₅, Y₂Mg₃Zn₃, YAg₂In, YAu₂In, YCd₃, YCe₃Au₈In₄, YCu₂In, YLaMg₆, YPd₂Bi, YPd₂In, YPd₂Pb,YPd₂Sb, YPd₂Sn, YbAu₂In, YbNi₂Sn, YbPd₂Pb, YbPd₂Sb, YbPd₂Sn, ZrAu₂In,ZrCo₂Al, ZrCo₂Sn, ZrCu₂Al, ZrNi₂Al, ZrNi₂In, ZrNi₂Sb, ZrNi₂Sn, andZrPt₂In.
 16. The coherent spin valve of claim 1, wherein at least one ofthe ferromagnetic electrodes is a semiconductor.
 17. The coherent spinvalve of claim 16, wherein the semiconductor is selected from the groupconsisting of (Ga, Mn)As and (Hg, Mn)Se.
 18. The coherent spin valve ofclaim 16, wherein the semiconductor is spin polarized by opticalpumping.
 19. The coherent spin valve of claim 1, wherein thenonmagnetic, electrically conducting layer material is a semiconductor.20. The coherent spin valve of claim 19, wherein the nonmagnetic,electrically conductive layer is graphite.
 21. The coherent spin valveof claim 1, further including a buffer layer between the nonmagnetic,electrically conducting layer and at least one of the ferromagneticelectrodes, the buffer layer configured to increase the spin injectionefficiency.
 22. The coherent spin valve of claim 21, wherein the bufferlayer is an oxide.